As the environmental, health and industrial impact of pollutants increases, it is becoming increasingly important to develop new methods for the rapid and efficient removal of a wide range of contaminants from polluted waters and other liquids. The invention is directed to a high efficiency method for the remediation of large quantities of liquid, operating at low to moderate ambient pressures, in order to reduce environmental or health risks or to purify the water for use in industrial processes. Moreover, this method reduces or eliminates the use of chemical additives. Rather, decontamination is achieved through the use of submerged liquid jets which trigger hydrodynamic cavitation events in the liquid. These cavitation events drive chemical reactions, by generating strong oxidants and reductants, efficiently decomposing and destroying contaminating organic compounds, as well as some inorganics. These same cavitation events both physically disrupt or rupture the cell walls or outer membranes of microorganisms (such as E. coli and salmonella) and larvae (such as Zebra mussel larvae), and also generate bactericidal compounds, such as peroxides, hydroxyl radicals, etc., which assist in the destruction of these organisms. Following disruption of the cell wall or outer membrane, the inner cellular components are susceptible to oxidation.
There are many means for removing contaminants and inclusions from liquids, including filtration, stripping, adsorption, absorption, and ion exchange. One technique employs oxidation of contaminants, in which chemical reactions are induced with oxidization agents to break the compounds down into simpler substances which, in turn, may also be oxidized. In the case of organic contaminants, the ultimate end products of oxidation reactions are typically nontoxic substances such as water and carbon dioxide. Thus, oxidation may completely destroy the contaminating substances, rather than merely removing them from the water for disposal elsewhere.
Oxidation reactions may be induced by a variety of means, such as the use of various chemicals, ozone, or supercritical water, or photochemical oxidation where ultraviolet radiation is used to produce hydroxyl radicals, which are strong oxidizing agents. These methods are often costly. Oxidation reactions also can be initiated by inducing hydrodynamic cavitation events in the solution, that is, by inducing the growth and rapid collapse of cavitation bubbles (also called cavities, microcavities or microbubbles) in the liquid. According to one theory, the generation of a "hot spot" (a local high temperature and pressure region) upon cavity collapse is responsible for dissociating the water molecules in aqueous liquids to produce hydroxyl radicals. Other oxidizing radicals may be formed in aqueous solutions as well as in non-aqueous environments. Oxidation reactions thus occur at the site of the collapsing cavity or bubble.
Systems using ultrasonically-induced cavitation have been found to promote a wide range of physical and chemical reactions and to be capable of at least partially oxidizing dilute aqueous mixtures of organic compounds. This may be achieved using ultrasonic horns to send a high intensity acoustic beam into the solution and excite microcavities. U.S. Pat. No. 4,076,617 (Bybel et al.) utilizes cavitation induced by acoustic means to create an emulsion of the waste material in water followed by application of ozone to oxidize the emulsified waste. U.S. Pat. No. 5,198,122 (Koszalka et al.) teaches the application of ultrasonic energy to contaminated liquids in the presence of oxidants. However, the efficiency of such ultrasonic devices is limited by achieving cavitation in the form of a cloud of cavitation bubbles only in a relatively small region near the surface of the ultrasonic source. Moreover, the efficiency of transfer of electric power into ultrasonic energy and then into the liquid itself is quite low, of the order of about 15%.
Other methods employ venturi flow to induce cavitation in contaminated aqueous solutions by relying on the pressure drop and subsequent pressure rise associated with flow through the venturi to cause cavitation bubble nuclei to grow and collapse. However, these methods are limited by their complexity and efficiency, and may require additional treatments, such as with chemical oxidizing agents, ultraviolet radiation, or both, to achieve the desired water purity. U.S. Pat. No. 4,906,387 (Pisani) and U.S. Pat. No. 4,990,260 (Pisani) teach first inducing cavitation in contaminated water which has been treated to provide hydroxyl free radicals and then irradiating the cavitated treated water with ultraviolet radiation. Cavitation is induced by passing the water through a cavitation critical flow constriction, shown in the figures to be a venturi-type constriction (that is, a cylindrical conduit of gradually decreasing and then gradually increasing inner diameter).
U.S. Pat. No. 5,326,468, U.S. Pat. No. 5,393,417, and U.S. Pat. No. 5,494,585 (the Cox patents) teach the production of oxidation by action of a cavitation venturi which is operated with a throat size and pressure drop to incur cavitation in the water. The Cox cavitation venturi comprises an inlet passage which converges in a cone, and a variable throat which is controlled by feedback from various sensors. The cavitation phenomenon which results in the formation and collapse of micro-bubbles is said to be contained in the expanding diameter outlet body of the venturi, the large end of which is essentially the same diameter as the inlet passage to the venturi. Sensors and programmable control feedback are used to adjust the throat of the venturi nozzle to optimize cavitation conditions. Oxidation is continued by the use of high energy ultraviolet radiation and/or hydrogen peroxide injection. The cavitation taught by Cox requires high velocities and energy in order for cavitation to occur as a result of the pressure drop generated in the liquid.
Submerged jet nozzles have been used to generate a highly concentrated and focused stream of cavitation in various fluids for the purpose of mechanically eroding, cutting, cleaning, or drilling into solid surfaces. See, for example, U.S. Pat. No. 4,508,577 (Conn et al.), U.S. Pat. No. 4,262,757 (Johnson et al.), U.S. Pat. No. 4,389,071 (Johnson et al.), U.S. Pat. No. 4,474,251 (Johnson et al.), U.S. Pat. No. 4,681,264 (Johnson et al.) and U.S. Pat. No. 3,528,704 Johnson, Jr.) which describe various fluid jets and their use for drilling, cleaning, cutting, and the like.
U.S. Pat. No. 3,528,704 discloses cavitating nozzles that utilize stem members of various configurations centrally positioned within the restricted orifice to further increase the velocity of the stream as it passes through the orifice. Additionally, as the stream passes over the stem member an evacuated core area is described as being formed which further reduces the pressure within the stream thus enhancing the formation of cavitation bubbles. One embodiment describes positioning flow rotating stator vanes within the nozzle chamber to impart a vortex movement to the stream as it leaves the nozzle. The nozzles described in this reference are taught to be useful in various drilling applications. U.S. Pat. No. 5,086,974 (Henshaw) decribes a cavitating jet nozzle for cleaning surfaces, which includes a free-floating pin received at a central position which lowers the pressure such that cavitation bubbles form in the liquid.
The use of rotating jet nozzles for cleaning and maintenance purposes is disclosed in U.S. Pat. No. 5,749,384 (Hayasi, et al.) and U.S. Pat. No. 4,508,577 (Conn et al.). The apparatus of Hayashi employs a driving mechanism capable of causing the jet nozzle itself to travel upward-and-downward, to rotate and swing. Conn et al. describe the rotation of a cleaning head including at least two jet forming means, for cleaning the inside wall of a conduit. Neither reference teaches or suggests rotational or swirling motion of the fluid jet itself.
U.S. Pat. No. 4,389,071 and 4,474,251 describe methods and apparatuses for oscillating the velocity of the liquid jet at a preferred Strouhal number within the range of about 0.2 to about 1.2. Such oscillation of the liquid jet is described as advantageously inducing a series of discrete vortices wherein cavitation occurs.
Each of the prior art methods and apparatuses for forming cavitating liquid jets require relatively high inlet pressures into the nozzles to induce cavitation in the liquid stream as it is ejected through the restricted orifices of the cavitating nozzle. The relatively high inlet pressures required in these prior art cavitating nozzles increases power consumption and limits the use of such cavitating nozzles to applications having a readily available source of high pressure fluid.
The present invention achieves increased oxidation by employing a mechanism for generating cavitation in the bulk of the liquid utilizing one or more swirling cavitating liquid jets which achieve cavitation at very high cavitation numbers. This enables maximization of the surface area of the cavities generated and of the volume of liquid subjected to cavitation and minimization of the power input required. This process can be made very efficient and also benefits from the fact that pumps are quite efficient in converting electric (or other) power into hydraulic power. Such a technology could be used for treatment of polluted water, groundwater, wastewater, industrial process water, and drinking water. It could be employed as a stand-alone process or as a part of a treatment train.